Achieving a photo-realistic scene with only built-in textures
(this response is more about the 'theory' of rendering planets and less about the technique)

The two principles I live by for making planets look better is minimal patterns and more detail. Starting with minimizing patterns, it's easy to make a planet that has a very visible repeating pattern, in this case it's most prevalent on the right planet. It's very obviously just a simple noise texture attached to a color ramp and it causes it to be very unnatural looking. Real planets generally do not have patterns like this on this scale, at least so visibly. I would recommend using a more complex mask for the color and bump. Essentially: combine different kinds of textures, with different scales. Using just one texture as you have here is too obvious, and adding more variety to the texture will make it much more natural. Blender has a few different procedural texture nodes to choose from, but no single one is really much good without mixing and balancing it against others. Try to aim for creating large "biomes" with different formations and colors in them. I'd recommend using larger scale textures as a mask for the different biomes, and textures with a smaller scale for the details. As for adding these details, definitely think about what sort of features you want your planet to have. Craters, mountains, ice caps, bodies of water, rivers, etc are all good examples of details that will bring your planet to life and make it more believable and interesting. Craters are easy to make with a Voronoi texture and color ramp. Ice caps can be mixed in using gradient textures. Rivers can be made with a noise texture and a color ramp that looks similar to this:

There is no written law for how to make different features and details, so play around with things and figure out what you like best!

With this image in particular, I personally would also work a bit on the lighting. The backlighting on the planets and vivid background picture aren't really photorealistic. I would make the background image much dimmer and remove any backlighting on the planets, as it wouldn't exist in real life (unless another star or planet was nearby to provide the light). This is an artistic choice, but to simulate reality better it is important to keep in mind how incredibly dim stars are, and no camera taking a picture of a planet like this would be able to capture surface detail and stars at the same time without one being far too dark or the other far too bright.

I hope this helped, and nice job on your render!